[Amps] Advice

[Dennis OConnor]

Yeah, I'll take that... Absolutely, the two tubes running at ~750 watts each will outlast by a sginificant margin (your lifetime) a single tube running at 1500 watts...  You are not beating up the oxide layer on the cathode nearly as much... 
And I'll toss out the statement (a bone to the dogs to chew on) that if you build it in push-pull it will last even longer because of the half cycle rest periods for the cathode give more time for ion exchange between the surface and deeper layers...
Actually, the life will be determined by how many times you light the fires and kick the wheels..  Everything I read suggests that the filaments passing through the 'poison band' during initial heating each time will eventually do the tube in... Some where between 600 and 1000 cycles is what I have seen opined...  That was, as I remember, for thoriated filaments... I do not know what numbers apply to non thoriated filaments...
 
There is a broadcast station that has kept a set of finals in stand bye,  running at mildly reduced filament  voltage, not dark heat,  for something like 30,000 or 40,000 hours... Had the tubes tested and they still met new specs..  Last thing I read the tubes are pushing 70,000 hours... Perhaps someone in the group with a better memory, or better sources, can elaborate...
denny / k8do